The classic transmission line theory that the fields cancel at all
points in space IS a convenient ARTIFICE but its NOT absolutely true.
When the conductors are at different distances from the observation
point, while their currents may be exactly out of phase when observed at
equal distances from the conductors, those fields from these currents
are not exactly out of phase (especially in the plane of the conductors)
because the propagation distance is different and the velocity of
propagation is only that of light, not instantaneous.
So is an inch spacing negligible at 50 Mhz? Is 5" spacing negligible at
10 MHz? I know former a former user of 1" spaced open line at 50 Mhz who
abandoned it because he found coax put more power to the antenna.
Lines spaced a quarter wave apart are the extremes of spacing (making an
antenna), yet the unbalance as seen on the plane of the elements is a
smooth function down to but not including conductor contact. The out of
phase condition due to the differences in distances to the two
conductors can never be exactly zero.
There is one other detail worthy of discussion of open wire feed lines
and that is: "Is feed line radiation harmful or a benefit?"
In some cases a significant bit of feed line radiation can make a hunk
of antenna wire more versatile, when that radiation from the feed line
happens to match a direction not worked by the antenna. It can be
harmful when it causes TVI, or telephone interference, or a hot
microphone.
73, Jerry, K0CQ
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Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.
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